Fix stdClass shape leakage across unrelated scopes

[SergioChan]

Summary

• prefer the current-scope variable union type for AST_VAR reads in inferStdClassShapePropertyType() when the local variable exists
• keep fallback behavior unchanged for non-variable expressions and unresolved variables
• add regression test 5918_stdclass_shape_scope_isolation.php for issue Phan incorrectly infers property type of newly created stdClass based on other assignments of properties on non-related stdClass instances #4789 so unrelated stdClass writes do not affect a fresh object's property type

Testing

• php -v (fails in this environment: php: command not found, so Phan tests could not be executed here)
• manual validation via new regression fixture + expected output asserting @phan-debug-var $var resolves to string

Related

Fixes #4789

[rlerdorf]

Fixed in #5486